1. Field of the Invention
This invention relates to a resist composition for plating, a method of producing the same and a printed wiring board using the same, and more particularly to a resist composition for plating with a fine structure having excellent photosensitive properties, resistance to alkali and heat resistance and an improved surface smoothness without causing development residue irrespectively of anchor depth, a method of producing the same and a printed wiring board using such a resist composition for plating.
2. Description of Related Art
Recently, miniaturization and high-speeding of electronic equipment are proceeding with the advance of electron industry. For this end, it is required that printed wiring boards and wiring board mounting LSI have high density and reliability based on a fine pattern.
As a method of forming conductors on the wiring board, there has lately been noticed an additive method wherein an adhesive is applied onto a surface of a substrate to form an adhesive layer, and a surface of the adhesive layer is roughened, and the substrate is subjected to an electroless plating to form conductors on the adhesive layer. According to this method, the conductor is formed by the electroless plating after the formation of a resist, so that the wiring having high density and pattern accuracy can be produced in a lower cost as compared with an etched foil method of forming the pattern through etching (subtractive method).
In order to produce the wiring having higher density and pattern accuracy in low cost by the above additive method, the selection of resists is particularly important. That is, it is necessary to use a resist composition capable of sticking to the roughened surface of the adhesive layer under a high follow-up action, and having a high resolution, and leaving no development residue on the considerably roughened surface after the development, and being very less in the amount of eluted ingredient even if the immersion in a high alkali bath is carried out at a high temperature for a long time.
As the resist composition satisfying the above properties, there have primarily been used ones obtained by skillfully combining a binder polymer having an average molecular weight of not less than several hundred thousand such as PMMA or the like with acryl oligomer having a photosensitive group and acryl monomer from a viewpoint of film-forming property, resistance to plating bath and heat resistance.
However, the resist composition containing the above polymer as a main ingredient has drawbacks that since the average molecular weight of the polymer is very large, the resulting cured resist is small in the crosslinking density and is swollen by intrusion of alkali when it is immersed in a high alkali bath for plating or the like at a high temperature. As a result, the conventional cured resist is largely eluted out in the plating solution by hydrolysis reaction with alkali in the plating treatment to degrade the plating solution and the properties of the plated film so that it has a problem in the resistance to alkali.
As a technique of improving the resistance of the cured resist to alkali, therefore, there is proposed a resist ink composition for electroless metal plating comprising an epoxy resin, an aromatic amine curing agent for epoxy resin and an imidazole curing agent for epoxy resin as a main ingredient and containing an organic solvent, a thixotropic agent, a filler and a chemical plating inhibitor (see JP-A-57-90072). This technique is particularly concerned with a resist ink protecting the conductor pattern in a semi-additive method. The cured product of such a resist ink is sufficiently durable to the immersion in an alkali bath of pH=12-13 (20.degree. C.) for a long time.
However, the resist ink according to this conventional technique is not sufficiently durable to the immersion in a high-temperature and high-alkali plating bath having pH of not less than 14 and a bath temperature of 80.degree. C. for not less than 5 hours, for example, for electroless tin-lead alloy plating other than the above electroless copper plating. Further, the resist ink is a thermosetting resin composition, so that it can not be applied to the printed wiring board formed through the additive method requiring photosensitive properties in the resist composition.
As a technique that a resin possessing both a thermosetting property and a photosensitivity is used in the resist composition, there is proposed a photosensitive resin composition wherein a half ester obtained by acrylating a part of epoxy group in novolac type epoxy resin is used as a base material (see JP-A-61-59447). According to this technique, there can be obtained a resist having excellent photosensitivity and heat resistance.
However, this technique has a problem that a cured resist sufficiently durable to the high-temperature and high-alkali bath having pH of not less than 14 and a bath temperature of 80.degree. C. can not stably be obtained without degrading the photosensitive properties.
In order to solve the above problems, therefore, JP-A-6-317904 discloses a resist composition for plating comprising an uncured novolac type epoxy resin in which a part of epoxy group is acrylated as a photosensitive resin ingredient and an imidazole curing agent as a curing agent and having excellent photosensitive properties, heat resistance and resistance to alkali. In this case, when an anchor depth is not more than 15 .mu.m, fine pattern having L/S=50/50 .mu.m or less can be obtained.
However, powder of the imidazole curing agent is actually used in the resist composition described in JP-A-6-317904, so that when the printed wiring board is actually manufactured by using such a resist composition, as shown in FIG. 1, the imidazole curing agent 2 existing at a crystal state projects from a side surface of the resulting resist pattern 3 formed by the exposure and development of the resist composition applied onto the adhesive layer 1 and hence there is caused a problem such as partial thinning or breaking of the wiring after the formation of the plated film. Furthermore, as the anchor depth becomes deeper, the development residue is generated and hence the fine pattern can not be obtained. Moreover, concave zones formed by deaeration after the application of the resist composition are observed on the surface of the cured resist, which is no problem when the pattern exceeds L/S=50/50, but if it is intended to provide the fine pattern, the plated film is precipitated in these concave zones to result in the occurrence of short-circuit between the wirings. And also, when the resist composition is liquid, if dusts or the like are adhered to the surface of the substrate, the resist composition is never applied to the dust existing portions.
In order to prevent the occurrence of the concave zone or the occurrence of non-applied portions through the dusts, there is proposed a method of adding an anti-foaming agent. In this case, the anti-foaming agent itself causes light scattering at a state of dispersing into the resist to create the development residue and hence there is caused a phenomenon of lowering the resolution of the resist for plating.